The continuous growth in human population and advances in industrialization comes　　 up with adverse effect on the environment.Because of the unavoidable impact environmental pollution has on livelihood of human kind, numerous research efforts have　　 been devoted on developing ways of cleaning up the environment in which use of photocatalytic oxidation technology was found to be the most promising method.Under　　 such circumstances, use of TiO2 was found to be the most suitable approach due to its high chemical stability, non-toxicity, robustness towards light corrosion as well as low cost.However, TiO2 has two main setbacks which are large band gap (allowing only the use of UV light for activation) and high photo induced charge carrier recombinations. The main focus of this work is to modify TiO2 in an effort to combat the said problems so as to render it usable at industrial level in environmental protection.　　 This thesis focuses on synthesis, characterization and photocatalytic application of visible light responsive TiO2 via simple and cost effective methods.The work was achieved in two ways which are non metal and transition metal single element doping of TiO2 and co-doping/modifying TiO2 with non metals and transition metals in order to enhance visible light absorption and promote photo induced charge carrier separation.　　 The details of this work are as follows;　　 1.Iodine doped titanium dioxide has been successfully prepared by simple hydrolysis of tetrabutyl titanate in the presence of iodic acid.The adopted method allowed for the production of spherical iodine doped titanium dioxide nanoparticles with varied amount of iodine content.Analysis by X-ray diffraction (XRD), Raman, transmission electron microscopy (TEM) as well as UV-Vis DRS revealed that titanium dioxide nanostructures were doped with iodine which existed in two different valence states I5+ and Iˉ.The iodine in the form of I5+ is believed to have been doped into the lattice whereas Iˉ was well dispersed on the surface of TiO2 probably as iodine adducts hence rendering it to be highly absorbing in visible light region.The I-TiO2 exhibited improved photocatalytic activity towards degradation of acid orange 7 (AO7), methyl orange (MO) and 2, 4-dichlorophenol (2, 4-DCP) under visible light over the pristine TiO2 prepared by the same method.High catalytic properties are attributed to iodine doping which led to high specific surface area, absorption in visible region as well as alleviation of charge carrier recombination.The most probable route undertaken in the degradation of AO7 is through indirect oxidation by the hydroxyl radicals.　　 2.Hydrothermal method was successfully employed to synthesis vanadium doped TiO2 with high visible light activity towards degradation of acid orange 7(AO7).Different characterization technologies employed reveal that the incorporation of vanadium ions into TiO2 renders it photocatalytically active under visible light irradiation.The increase in the content of doped vanadium ion was accompanied by reduction in the band gap energy (from 3.01 eV to 2.60 eV) in Degussa P25 to 3％ V-TiO2 respectively.This enabled TiO2 to be effective in visible light absorption.Furthermore vanadium ions improved the crystallinity of TiO2 which is one of the good properties for enhanced photocatalytic activity due to reduced amount of defects that normally acts as charge carrier recombination centers.The sample with 3％V exhibited highest activity under the set conditions.　　 3.Visible light active nanostructured TiO2 particles co-doped with vanadium and　　 iodine were prepared with a simple hydrothermal method.The physical structures, optical and chemical status of the products were evaluated by XRD, Rarnan, TEM, N2 physical adsorption/desorption isotherms, UV-DRS, FTIR, PLS and XPS.The results revealed that vanadium was doped in the lattice as V4+ and V5+ while iodine was doped into the surface as 17+ and I-oxidation states.It was found that the red shifting of the absorption edge of TiO2 was mainly induced by vanadium lattice doping while surface doped iodine species enhanced visible light absorption as well as facilitating electron-hole pair separation.The presence of iodine in the co-doped samples led to reduced particle size but induced low porosity.Cooperate action of vanadium and iodine on the physiochemical and optical properties of TiO2 enhanced its photocatalytic activity towards degradation of AO7 under visible light irradiation.Amongst the prepared series of samples, the sample with nominal iodine amount of 7％ exhibited highest activity.　　 4.Bismuth modified nitrogen doped TiO2 nanoparticles have been successfully　　 prepared by two step synthesis route which includes hydrothermal and impregnation　　 hydrolysis method.Samples were characterized using XRD, N2 physical adsorption,　　 (TEM), UV-Vis DRS, FTIR, Raman, XPS and PLS technologies.The preparatory method afforded the production of well crystallized spherical Bi modified N-doped TiO2　　 nanoparticles with varied amounts of Bi content.XRD analysis results reveal that Bi exists as rare metastable Bi20TiO32 which started to surface at Bi loading content of 7mol％ in relation to Ti iota.All Bi modified N-TiO2 samples exhibited higher photocatalytic activity toward degradation of 2, 4-DCP over N-TiO2 under visible light inradiation.The sample with 10％ composition of the Bi20TiO32 exhibited the highest activity.The superior photocatalytic performance of 10％ Bi/N-TiO2 is attributed to high visible light absorption as well as effective charge carrier separation.Therefore, the role of Bi species in the N-TiO2 is improvement of visible light harvesting, improving sample crystallinity and facilitation of charge carrier separation hence alleviating electron-hole recombination.　　 5.Bismuth and Boron co-doped TiO2 nanoparticles were successfully prepared by a modified sol-gel method.The products were characterized with various spectroscopic and analytical techniques to determine their structural, morphological, light absorption and photocatalytic properties.The results reveal that all the samples consist of highly crystalline anatase with mesoporous structures.The experimental results further indicatethat Bi and B species have been doped into the crystal lattice of TiO2 with Bi substituting Ti in the form of Bi3+ and B doped in the form of substitutional and interstitial B.The presence of Bi species facilitated the incorporation of B into the crystal lattice of TiO2. XRD and TEM analysis show that all the dopants (B and Bi) have the ability to inhibit particle growth of anatase TiO2 with more inhibition exhibited by Bi.Compared to pure TiO2, B and Bi singly doped TiO2; Bi-B co-doped samples showed better activities for degradation of Acid Orange 7 (AO7) and 2, 4-dichlorophenol under visible light irradiation.The highest activity is observed for 3％ Bi-B-TiO2 calcined at 450℃.The superior performance of this sample is ascribed to the high surface area, ability to absorb in visible light, efficient charge separation as well as improved e-transfer associated with　　 the cooperate effects of appropriate amounts of B and Bi in co-doped sample.Bi species are found to play a pivotal role in the co-doped samples.Superoxide radicals are the most reactive species in degradation of AO7 over 3％ Bi-B-TiO2 under visible light irradiation.